Over the past several years there has been a simultaneous growth in the wireless communication product market, leading to an increasing use of WLAN (wireless local area network). One particularly important thing was the publication of IEEE 802.11 WLAN protocol in 1997. The WLAN protocol not only provides many novel features of WLAN communication but also provides a solution for communicating among various wireless communication products. This is a milestone for the development of WLAN. The WLAN protocol also ensures a solution of performing a single chip by means of core device and greatly reduces the cost of employing wireless technique. As a result, WLAN is readily adapted to employ in a variety of wireless communication products.
When a wireless communication product is transmitting data wirelessly in a high power state, electromagnetic wave is generated and may in turn cause EMI (electromagnetic interference). For solving this problem many modern countries have stipulated some related rules in order to limit the import and the use of wireless communication products not complying with the related rules. In order to comply with the rules, many designers and manufacturers of wireless communication product have to redesign the control circuit of the wireless communication product. Typically, they add a filter next to a power amplifier for eliminating the harmonic spurious of high frequency. As an end, the harmonic of high frequency of the wireless communication product being produced is adapted to comply with the EMI correction rule. However, the filter, mounted in the control circuit of the wireless communication product being designed and manufactured by the manufacturers, is responsible for only eliminating the harmonic of double frequencies generated by the wireless communication product. This is best illustrated in the graph of FIG. 1 in which the measurement result of filtering a wireless communication product having a signal transmission frequency of 5.5 GHz is shown. An insertion loss about 0.1 dB, a reduction about 35.8 dB at the harmonic of double frequencies of 11 GHz, and a poor reduction about 7.17 dB at the harmonic of triple frequency of 16.5 GHz are also shown. A high frequency filter having a higher insertion loss is required if it is desired to eliminate both the harmonic spurious of double frequencies and the harmonic spurious of even higher frequency at the same time. However, the high frequency filters are expensive and bulky, thus contradicting the trend of designing a lightweight, compact, and portable electronic products. In view of the above, the installation of the high frequency filter is not desired due to the greatly increased manufacturing cost and size.
The harmonic generated by a wireless communication product in transmitting high power signals will be the major reason causing the product difficult to pass though the EMI test being conducted. Particularly, the harmonic of double frequencies and the harmonic of higher frequency of the signal transmission frequency are even harder to pass though the EMI test.
Thus, it is desirable among many designers and manufacturers to design wireless communication products to comply with the stipulated rule without greatly increasing the manufacturing cost and the size in order to overcome the above drawbacks of the prior art.